Apparatus for balancing a bucket centrifuge rotor

ABSTRACT

Apparatus for automatically balancing a bucket centrifuge wherein a weight constrained to move only along an axis that intersects the center of rotation of the centrifuge is placed within each bucket. The weight in a bucket is free to move when the bucket is empty but prevented from moving when the bucket contains a sample container. The sample and sample container and the mass of the weight are regulated so that the moment of the weight in its radially outermost position equals the moment of the sample, sample container, and the weight in its radially innermost position.

United States Patent Hanlrey APPARATUS FOR BALANCING A BUCKET CENTRIFUGE ROTOR [75] Inventor: James Hankey, Milton, Mass.

[73] ASSIgnEFDaEnEfi jorpoiat ion, Needham,

Mass.

[22] Filed: Apr. 14, 1971 [21] Appl. No.1 133,947

FOREIGN PATENTS OR APPLICATIONS Germany 233 1 c 862,59] 3/l96l Great Britainl 74/573 Primary Examiner-George H. Krizmanich Att0rneyKenway, Jenney & Hildreth [57] ABSTRACT Apparatus for automatically balancing a bucket centrifuge wherein a weight constrained to move only along an axis that intersects the center of rotation of the centrifuge is placed within each bucket. The weight in a bucket is free to move when the bucket is empty but prevented from moving when the bucket contains a sample container. The sample and sample container and the mass of the weight are regulated so that the moment of the weight in its radially outermost position equals the moment of the sample, sample container, and the weight in its radially innermost position.

2 Claims, 4 Drawing Figures PATENIED UB1 FIG.

INVENTOR JAMES J. HANKEY FIG. 3

I 5 z 5 2 ATTORNEYS APPARATUS FOR BALANCING A BUCKET CENTRIFUGE ROTOR This invention relates to a method and apparatus for balancing a rotating bucket centrifuge rotor.

Centrifuge rotors adapted to separate liquid samples based on density have a plurality of compartments from which hang detachably-mounted sample containers. The sample containers are mounted so they can swing from a depending vertical position to a horizontal position responsive to centrifugal forces. Many times, the number of samples being processed is less than the available compartments thereby resulting in the rotor being unbalanced about its central vertical axis. Thus, in automatic chemical analysis systems, samples are centrifuged in a preset test sequence to maintain proper sample identification and the number of samples processed may be less than the centrifuge compartments available. Under these conditions, the technician cannot redistribute the sample container to attain a balanced load in the rotor. When the rotor is spun, this imbalance generates vibrations that damage the motor bearings and may lead to bearing failure. In addition, the vibrations reduce the safe operable speed of the rotor and agitate any interface formed between constituents of the sample being separated, thereby causing inadequate separation. It would be highly desirable to provide a simplified means for preventing rotor imbalance that can be implemented automatically and is operable regardless of the number of samples being processed.

The present invention provides a rotor having buckets into one wall of which are placed weights adapted to move radially when the bucket moves from a vertical to a horizontal position when the rotor is spun and when no sample container is placed in the compartment. Means are provided for preventing the weight from moving when a sample container is placed in the compartment. The position, configuration and weight of the counter-weight and sample system are defined by the following equation.

in I J w wherein W is the weight of the counter-weight. W, is the weight of the sample container and sample. R is the distance from the center of rotation of the rotor to the center of gravity of a sample and sample container. and

'AR is the displacement of the counter-weight along the major axis of the housing when the rotor is spun. The invention will be more fully described with reference to the accompanying drawings.

FIG. 1 is a top view of the rotor of this invention. FIG. 2 is a vertical cross-sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a vertical cross-sectional view of the balancing means of this invention.

FIG. 4 is a top view, in partial cross-section of the a balancing means of FIG. 3.

The centrifuge rotor 1 is provided with a plurality of equally spaced recesses 2 into which are placed buckets 3. The buckets 3 are privoted on arms 4 which fit into the recesses 2 and into opposing outside surfaces of the buckets 3 in a manner well known in the art. The

balancing means 5 is placed in a recess 6 formed on the inside surface of each bucket 3. When a sample is to be centrifuged, it is placed in recess 7 of the bucket 3 and when the centrifuge 1 is rotated, "the buckets pivot on arms 4 to move radially outward from a generally vertical position to a generally horizontal position.

When a sample container is placed in recess 7, it forces spring-loaded plate 8 toward the center of rotation of the rotor l. The plate 8 is slidably mounted on pins 9 and when forced toward the center of rotation of the rotor, the recess 12 fits around arm 10 of weight 14. Thus, when the plate 8 is in the: radially inward position, weight 14 cannot move out of compartment 15 located above plate 8.

On the other hand, when no sample container is placed in the recess 7, the plate 8 being spring biased outwardly by springs 16, does not contact or constrain weight 14 and when the rotor 1 is spun, and the buckets 3 moved into a generally horizontal position, weight 14 moves in a radially outward direction into compartment 20 so that it contacts a shoulder 21 in the bottom of compartment 20. When in this latter position, the weight 14 is in its radially outermost position.

The equation above is explained with reference to this Figure wherein W,,, is the weight of part 14, W, is the total weight of the sample container placed in the bucket plus sample, R, is the distance from the center of rotation of the rotor to the center of gravity of the sample container and sample, and AR is the displacement of weight 14 from the top of compartment 15 to the bottom of compartment 20. In addition, the axis upon which the weight 14 moves when the bucket 3 is spun must align with the center of rotation of the rotor.

When at least one recess 7 is empty and at least one recess 7 contains a sample container and the rotor is spun, the buckets 3 will swing on arms 4 so that the bottom surfaces of the buckets 3 are in the radially ouler most position. In the recesses 7 containing a sample container, plate 8 is forced into contact with arm 10 under weight 14 by the sample container and the weight 14 is prevented from moving out of compartment 15.In the recesses 7 not containing a sample con tainer, springs 16 constrain plate 8 away from arm 10 and weight 14 so that weight 14 is free to move past recess 12 and to the bottom of compartment 20.

The housing 21 for the weight 14 can have any desired shape so long as it is complementary with the recess in the bucket 3 into which the sample system is to be placed. Furthermore, the configuration of the rotor, the bucket and the cavities need. not be restricted to those illustrated in the drawings. For example, the rotor can have more than four compartments and can have a different shape. Also, any means can be provided for constraining the weight when a sample container is placed in a bucket and allowing the weight to move radially when no sample container is placed in a bucket. Thus, manually operated means located on the top surface of the rotor can be adapted to constrain or allow free movement of the weight. All that is necessary is that the weight adapted to move in a radial direction be constrained on doing so when a sample container is placed into the bucket but is free to move when no sample is placed in the bucket.

I claim:

1. Apparatus for automatically balancing a bucket centrifuge rotor having a plurality of equally spaced buckets during rotation of the rotor comprising a weight in each bucket constrained to move in a radial R is the distance from the center of rotation of the direction along an axis that intersects the center of rorotor to the center of gravity of a sample and samtation of said rotor and means for preventing movepie Container in a bucket ment of a weight in a bucket containing a sample conand tainer and for permitting movement of the weight in a bucket not containing a sample container, the relationship of the weight, sample and sample container being defined by the equation:

constraining and permitting movement of the weight W /W, n/A w 10 comprises means biased away from contact with the wherein weight and adapted to be forced into contact with the W is the weight of the counter-weight. weight by a sample container placed in a bucket. W, is the weight of the sample container and sample.

AR is the displacement of the weight along the major axis of the housing when the rotor is spun. 2. The apparatus of claim 1 wherein the means for 

1. Apparatus for automatically balancing a bucket centrifuge rotor having a plurality of equally spaced buckets during rotation of the rotor comprising a weight in each bucket constrained to move in a radial direction along an axis that intersects the center of rotation of said rotor and means for preventing movement of a weight in a bucket containing a sample container and for pErmitting movement of the weight in a bucket not containing a sample container, the relationship of the weight, sample and sample container being defined by the equation: Ww/Ws Rs/ Delta Rw wherein Ww is the weight of the counter-weight. Ws is the weight of the sample container and sample. Rs is the distance from the center of rotation of the rotor to the center of gravity of a sample and sample container in a bucket. and Delta Rw is the displacement of the weight along the major axis of the housing when the rotor is spun.
 2. The apparatus of claim 1 wherein the means for constraining and permitting movement of the weight comprises means biased away from contact with the weight and adapted to be forced into contact with the weight by a sample container placed in a bucket. 